Annealing of Alkaline-Earth Metal Polymetaphosphate Powders (I) Crystallisation and Sintering Processes
Aniorphous barium, strontium, calcium and magnesium polyriietaphosphate powders (MP,O,),, m = 20 were prepared by dehydration of the corresponding polyinetaphosphate hydrate precipitates. These powders were annealed by different continuous and isothermal heat treatments over the temperature range 450" to 700 "C, the glass transition teniperatures T', to above (Y' , + 120) "C. The morphologies a t differcnt degrees of crystallisation were studied by scanning electron microscopy. For the main crystallisation process (ten to sixty-seventy percent crystallisation), the powder particles retained their original pea-pod form; then after seventy percent crystallisation, these rrystallised particles sintercd laterally to lozenge-shaped twin-hexagonal crystals of lengths 0.5 to 3 prn.
Differential thermal analysis confirmed that a markedly exothermic crystallisation process (overall enthalpy changes from about 30 to 45 k J riiol-l) was occurring within the powder particles. Crystallisation rates varied froin < 0.005 min-l at teinpcratures near 1 ' , to > 0.5 miii-' a t higher temperatures; the activation energies for this process varied froin 360 to 660 k J mol-l.
The completely annealed crystals wore studied by scanning electron inicroscopy, X-ray diffraction and further differential thermal analysis to 1000 "C. The X-ray diffraction d value patterns, the fusion temperatures and the enthalpirs of fiision were all in close agrcernent with the literature values for the corresponding beta alktdine-earth nictal pol yriie t ;Lphospha t es prcparc'd by rnel t cry st a1 1 isat ion.